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SOMERVILLE - Following a half-dozen sexual assaults reported by women in recent months, a 22-year-old man has been charged with three of them and is being held without bail pending a dangerousness hearing scheduled for next week.

At an arraignment yesterday in Somerville District Court, Judge Neil Walker ordered Alfredo R. Posada of Somerville held pending the dangerousness hearing, scheduled for Tuesday. Posada, who was arrested Thursday after being identified in a police lineup, pleaded not guilty. No one has been charged in the other three attacks.

The assaults Posada is charged with occurred Sept. 10 and Sept. 24 in Somerville, and Oct. 22 in Medford, according to court records. In each incident, Posada has been charged with one count of assault to rape and two counts of indecent assault and battery on a person 14 or over, according to court records.

Assistant District Attorney Ceara Mahoney said in court that Posada groped the three women in the late-night attacks, following and then accosting each of his victims. In each case, the victim was walking alone, Mahoney said.

Prosecutors said in court that Posada suffers from schizophrenia, and a relative of his said he had recently stopped taking his medication for the illness.

His cousin Wendy, who declined to give her last name, said Posada had stopped taking medication because it interfered with his ability to work and live a normal life. She said Posada had been hospitalized in the past couple of months in connection with the schizophrenia.

“[He] stopped taking his medication, but he hasn’t relapsed,’’ Wendy said in an emotional interview outside the courthouse after the arraignment yesterday. “The doctor told us he could relapse.’’

The most recent of the sexual assaults was reported about 1:30 a.m. Thursday, prompting Somerville police to promise stepped-up patrols, advising women to be aware of their surroundings and to avoid dark or remote areas.

Posada’s cousin said that Posada had had contact with the police in recent days, and that he had agreed to participate in the lineup, but she was unsure of how or when officers first reached him. When she last spoke with Posada on Wednesday, she said, the two discussed his agreement to go to the police station.

Officers from Cambridge, Medford, Tufts University, the FBI, and MBTA Transit Police have collaborated to search for suspects in the assaults, according to a statement by Somerville Deputy Police Chief Paul Upton.

In court, prosecutors described the attacks in detail, including the Oct. 22 assault, in which a female Tufts University graduate student who had left a party was followed by a man dressed in black whom she later identified to police in the lineup as the suspect. Mahoney said her attacker forced the woman to the ground along College Avenue in Medford in a frenzy, groping her above and beneath her clothing.

“It was almost as if he could not touch her fast enough,’’ Mahoney said.

Posada, a native of El Salvador who speaks limited English, came to the United States several months ago to look for work, his cousin said.

“He is quiet, hard-working, very timid,’’ she said. “I think what [the prosecutors] said is completely out of the question.’’

Immigration and Customs Enforcement did not immediately respond to a call last night inquiring about Posada’s legal status as an immigrant.